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CAIE A-Level Chemistry 22.1.1 Analysing Infrared Spectra

Practise reading a simple infrared spectrum to identify functional groups from characteristic absorption ranges.

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

Exam points

  • read each marked wavenumber and match it to the data-table range before assigning a bond
  • support a functional-group claim with all required peaks, such as ester C=O and C–O absorptions
  • compare expected and observed peaks to assess purity or distinguish candidate molecular structures

22.1.1—Analyse an infrared spectrum of a simple question 1

[Maximum number: 3]

Organic compounds can be distinguished using chemical tests and analytical techniques.

Question (a)

(a)

Organic compound E contains three carbon atoms. E reacts with cold dilute acidified KMnO4(aq)\mathrm{KMnO}_{4}(\mathrm{aq}) to form a single compound F with Mr=154.9M_{\mathrm{r}}=154.9.
Fig. 3.1 shows the infrared spectrum of E.
Fig. 3.2 shows the infrared spectrum of F.

Fig. 3.1

Fig. 3.1

Fig. 3.2

Fig. 3.2

Table 3.2

Table 3.2

Both spectra show absorptions between 2850 and 2950 cm−12950 \mathrm{~cm}^{-1} owing to C-H bonds in each molecule.

[ 3 ]

Question (i)

(i)

Use the two infrared spectra and Table 3.2 to identify the functional group present only in E.
Explain your answer, referring only to absorptions at frequencies greater than 1500 cm−11500 \mathrm{~cm}^{-1}.
functional group explanation

[ 1 ]

Question (ii)

(ii)

Use the infrared spectrum of F to identify the functional group formed when E reacts with cold dilute acidified KMnO4(aq)\mathrm{KMnO}_{4}(\mathrm{aq}).
Explain your answer, referring only to absorptions at frequencies greater than 1500 cm−11500 \mathrm{~cm}^{-1}.
functional group explanation

[ 1 ]

Question (iii)

(iii)

Use the information in 3(c) to suggest a structure for E.

[ 1 ]
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